Smash your head against a tree

Bill Chaisson
Of a Feather
Last Sunday I was hiking up the White Pine Trail, a segment of the Sunapee-Ragged-Kearsarge Greenway, and the woods were very, very quiet, except for the sound of snow ticking through the bare branches. I thought I was hearing the echo of the crunch of my boots and stopped to make sure. Silence. But when I resumed my pace the “echoes” were out of sync with my steps. I stopped again. Thuk. Thuk. Pause. Thuk.

After years and years of listening to the sound of woodpeckers pounding, I have developed a (not perfect) sense of which species it is just by the pitch. The smaller the woodpecker, the higher the pitch. This time I immediately predicted I was hearing a hairy woodpecker. Because there were ashes in the forest, I searched the trunks of those first because they are all dying due to the emerald ash borer. I found two hairy woodpeckers following each other around, occasionally stopping to make an exploratory tattoo.

It is easier to guess the species in winter. Only the downy, hairy, red-bellied, and pileated stick around through the cold weather. I don’t know why some woodpeckers migrate and some don’t. The red-headed woodpecker is in the same genus as the red-bellied, but it migrates south each winter and the red-bellied stays put. Both species have diverse diets and the red-headed woodpecker is even one of the four (out of 198) woodpecker species that stores food.

No matter how varied their diets may be, we think of this family as a group of insect eaters because they have evolved such specialized apparatus and behaviors to forage for invertebrate prey. Let’s just say the bills are chisel-like and consider the feet, the tail, the head, and the tongue.

The family Picidae are zygodactylous. That is, their first and fourth toes are oriented to the rear. The ancestral condition in birds, which is seen among theropod dinosaur fossils, is to have four toes pointing forward (although with just three touching the ground). Zygodactyly has evolved separately three times among the Aves—the other families are parrots and cuckoos—for similar evolutionary reasons. Some birds, like owls and ospreys, are able to rotate one toe to the back when they want to get a better grip. In the case of those raptors it is to hold onto prey. But the mousebirds of Sub Saharan Africa rotate their toes at will in order to hang upside down while foraging.

Generally speaking, zygodactyly has been described as an adaptation for clinging. In woodpeckers it allows them to cling to vertical surfaces and to hang upside down. That all four toes are not strictly necessary to crawl up vertical tree trunks is proven by the genus Picoides. The three-toed woodpeckers have lost the hallux (digit I) but still forage in woodpecker fashion.

Anisodactyl (II, III, IV toes forward, I back) birds like creepers, nuthatches, and black-and-white warblers are also able to hop on tree trunks, but only the creeper shares the picids’ stiffened tail feathers that serve as a brace while the birds probe for food. The central feathers are the stiffest and often the longest. The importance of these feathers can be seen in their molting pattern. Most birds shed their central tail feathers first, but woodpeckers and creepers shed the outer tail feathers first so they will be able to feed while the strongest feathers grow back.

In a July 2022 article in New Scientist, Christa Lesté-Lasserre interviewed Sam van Wassenbergh of the University of Antwerp. Van Wassenbergh has shown that the spongy bone between the birds’ brains and beaks does not cushion the brain against the impact of the hammering beak. It is the other way around: it stiffens the skull in order to make it a more effective hammer. By analyzing high-speed video, he and his colleagues saw that in the milliseconds after impact, the bird’s eyes and head slowed down at the same rate as the bill. That is, there was no cushioning effect to absorb the force of impact. Instead, the sponginess of the bone in the head is an adaptation that decreases the weight of the head; lightness is important for flying birds.

Van Wassenbergh also found that woodpeckers do not need protection from brain damage. Their brain is surrounded by a fluid-filled sac. They would need to strike with twice the force with which they are capable or something four times as hard as wood in order to injure their brains.

While some woodpeckers eat almost anything, three-toed woodpeckers specialize in bark beetles from fire-damaged trees. Their head bones, leg structure, and muscles are all modified in order to maximize the force of the blow they are able to deliver.

Finally, woodpeckers have specialized tongues they are able to insert into their excavations and extract their prey. Rebecca Heisman, writing for the American Bird Conservancy, does a nice job of summarizing the adaptations. In humans the hyoid bone is U-shaped and nestled under the tongue between the lower mandibles. In woodpeckers it is in the upper mandible (beak). It emerges to pass between the eyes, splits in two, wraps all the way around the back of the head, and joins into one piece again in the lower mandible. The tongue muscles are wrapped around this bone and when they contract the tongue projects forward.

Woodpeckers’ tongues are exceptionally long. Although the proportions vary from species to species, the tongue can be one-third the length of the entire bird. Northern flickers are the North American species with the longest tongue; it can protrude two inches past the end of its beak. Those of three-toed woodpecker are the shortest.

Flicker tongues are smooth and sticky for extracting ants from their holes. Pileated woodpecker tongues are relatively short and barbed to remove insects from crevices. Sapsuckers have bottle-brush-like structures that absorb sap (and the insects trapped in them) by capillary action.

— Bill Chaisson has been birdwatcher for over 50 years. Contact him at [email protected]. These columns are archived at shinhollow.wordpress.com.